1. Field of the Invention
The present invention relates to suspended single crystal silicon structures micromachined in a standard CMOS integrated circuit process, and the micromachining method itself. More specifically, the invention relates to suspended single crystal silicon structures useful as temperature sensitive transducers and as low-power temperature-controlled circuitry; and to a method of micromachining an integrated circuit without the need for additional masking layers using an electrochemical post-processing etch.
2. Related Art
There are many types of transducers, such as vacuum sensors, gas flow sensors, infrared detectors, and AC power converters, which operate based on the detection of a localized temperature difference in an integrated circuit context. Examples of these transducers are given in Y. Xu, R. Huang, and G. Rigby, "A Silicon-Diode-Based Infrared Thermal Detector Array," Sensors and Actuators A, Vols. 37-38, pp. 226-30 (1993); H. Baltes and D. Moser, "CMOS Vacuum Sensors and Other Applications of CMOS Thermopiles," Transducers '93 Dig. Tech. Papers pp. 736,41 (New York 1993); D. Jaeggl, H. Baltes, and D. Moser, "Thermoelectric AC Power Sensor by CMOS Technology," IEEE Electron Device Letters, Vol. 13, No. 7, pp. 366-68 (1992); A. van Herwaarden D. van Duyn, B. Oudheusden, and P. Sarro, "Integrated Thermopile Sensors," Sensors and Actuators A, vol. 21-23, pp. 623-30 (1989); J. Choi and K. Wise, "A Silicon-Thermopile-Based Infrared Sensing Array for Use in Automated Manufacturing," IEEE Trans. Electron Devices, Vol. ED-33, No. 1, pp. 7-79 (1986); and D. Moser, R. Lenggenhager, and H. Baltes, "Silicon Gas Flow Sensors Using Industrial CMOS and Bipolar IC Technology," Sensors and Actuators A, Vols. 25-27, pp. 577-81 (1991). These transducers benefit from the temperature sensor having good thermal isolation, high temperature sensitivity, and small thermal mass.
There are various ways to measure temperature in an integrated circuit process. The first is through the use of thermopiles. Thermopiles are series of junctions between dissimilar materials which develop a voltage across them proportional to a temperature gradient. As described in D. Jaeggl, H. Baltes, and D. Moser, "Thermoelectric AC Power Sensor by CMOS Technology," two such dissimilar materials used in circuit processing are aluminum and polysilicon, which have a temperature sensitivity of around 60 .mu.V/.degree.C. per junction.
Another way to measure temperature is to exploit the temperature dependence of the forward voltage drop across a diode, which is 2000 .mu.V/.degree.C. This second method detects absolute temperature, and not strictly a temperature gradient.
For sensitive temperature transduction, the sensor needs to be thermally isolated from the rest of the silicon substrate. One way to provide such thermal isolation has been to create CMOS or bipolar compatible thermal transducers using post-processing techniques, as described in J. Suehle, R. Cavicci, M. Gaitan, and S. Semancik, "Tin Oxide Gas Sensor Fabricated Using CMOS Micro-Hotplates and In-situ Processing," IEEE Electron Device Letters, Vol. 14, No. 3, pp. 118-20 (1993); Y. Xu, R. Huang, and G. Rigby, "A Silicon-Diode-Based Infrared Thermal Detector Array," Sensors and Actuators A, Vols. 37-38, pp. 226-30 (1993); H. Baltes and D. Moser, "CMOS Vacuum Sensors and Other Applications of CMOS Thermopiles," Transducers '93 Dig. Tech. Papers pp. 736,41 (New York 1993); D. Jaeggl, H. Baltes, and D. Moser, "Thermoelectric AC Power Sensor by CMOS Technology," IEEE Electron Device Letters, Vol. 13, No. 7, pp. 366-68 (1992); A. van Herwaarden D. van Duyn, B. Oudheusden, and P. Sarro, "Integrated Thermopile Sensors," Sensors and Actuators A, vol. 21-23, pp. 623-30 (1989); N. Swan, A. Nathan, M. Sharns, and M. Parameswaran, "Numerical Optimisation of Flow-Rate Microsensors Using Circuit Simulation Tools," Transducers '91 Dig. Tech. Papers, pp. 26-29 (New York 1991); and M. Parameswaran, H. Baltes, L. Ristic, A. Dhaded, and A. Robinson, "A New Approach for the Fabrication of Micromechanical Structures," Sensors and Actuators, Vol. 10, pp. 289-307. These devices are either realized with suspended oxide membranes, as described in J. Suehle, R. Cavicci, M. Gaitan, and S. Semancik, "Tin Oxide Gas Sensor Fabricated Using CMOS Micro-Hotplates and In-situ Processing", IEEE Electron Device Letters, Vol. 14, No. 3, pp. 118-20 (1993), or cantilever-style single crystal silicon structures, as described in Y. Xu, R. Huang, and G. Rigby, "A Silicon-Diode-Based Infrared Thermal Detector Array," Sensors and Actuators A, Vols. 37-38, pp. 226-30 (1993).
J. Suehle, R. Cavicci, M. Gaitan, and S. Semancik, "Tin Oxide Gas Sensor Fabricated Using CMOS Micro-Hotplates and In-site Processing," IEEE Electron Device Letters, Vol. 14, No. 3, pp. 118-20 (1993); Y. Xu, R. Huang, and G. Rigby, "A Silicon-Diode-Based Infrared Thermal Detector Array," Sensors and Actuators A, Vols. 37-38, pp. 226-30 (1993); H. Baltes and D. Moser, "CMOS Vacuum Sensors and Other Applications of CMOS Thermopiles," Transducers '93 Dig. Tech. Papers pp. 736,41 (New York 1993); D. Jaeggl, H. Baltes, and D. Moser, "Thermoelectric AC Power Sensor by CMOS Technology," IEEE Electron Device Letters, Vol. 13, No. 7, pp. 366-68 (1992); A. van Herwaarden D. van Duyn, B. Oudheusden, and P. Sarro, "Integrated Thermopile Sensors," Sensors and Actuators A, vol. 21-23, pp. 623-30 (1989); and I. Choi and K. Wise, "A Silicon Thermopile-Based Infrared Sensing Array For Use In Automated Manufacturing," IEEE Trans. Electron Devices, Vol. ED-33, No. 1, pp. 72-79 (1986) disclose that the thermally isolated structures can sense a localized change in temperature, making them useful for applications such as infrared detection, gas flow monitoring, and AC power measurement. While oxide membranes are useful for building thermopiles and polysilicon heating resistors, they do not allow for the formation of active devices such as diodes or transistors. Silicon cantilevers, while providing diodes for highly sensitive temperature transducers, are connected to the substrate with low thermal resistance single crystal silicon. Moreover, the processes used for the fabrication of these cantilevers either are not CMOS compatible (Y. Xu, R. Huang, and G. Rigby, "A Silicon-Diode-Based Infrared Thermal Detector Array," Sensors and Actuators A, Vols. 37-38, pp. 226-30 (1993) or require backside alignment (A. van Herwaarden, D. van Duyn, B. Oudheusden, and P. Sarro, "Integrated Thermopile Sensors," Sensors and Actuators A, vol. 21-23, pp. 623-30 (1989)). In either case protection of the metalization layers is required during the electrochemical etch.
A related problem is the provision of a voltage reference that is stable over temperature variation. Such a voltage reference is an important component of most data acquisition systems. Examples are described in T. Brooks et al., "A Low-Power Differential CMOS Bandgap Reference," ISSCC Digest of Technical Papers, pp. 248-49 (February 1994); and M. Ferro et al., "A Floating CMOS Bandgap Voltage Reference for Differential Applications," IEEE J. Solid-State Circuits, Vol. 24, pp. 690-97 (June 1989). Improved stability has been achieved through bandgap curvature compensation techniques and by regulating the temperature of a zener diode reference with an on-chip heater. The "LTZ1000 data sheet," Linear Technology Corporation, Milpitas, Cal.; R. Dobkin, "Monolithic Temperature Stabilized Voltage Reference with 0.5 ppm/.degree. Drift," ISSCC Digest of Technical Papers, pp. 108-09 (February 1976); and D. Laude et al., "5V Temperature Regulated Voltage Reference," IEEE J. Solid-State Circuits, Vol. 15, pp. 1070-1075 (December 1980) describe achieving very stable performance, ranging from 0.3 ppm/.degree.C. down to 0.05 ppm/.degree.C., using the heated substrate method. The disadvantages of this approach have been high power consumption (up to 800 mW), slow warm-up time (3 seconds) and specialized non-CMOS processes. It is to the solution of this problem and those described above to which the present invention is directed.